Possible minor spoilers below.

> So the question remains, for me, is whether the Druid class is useful in a character progression?
>
> Given that a Wizard or Sorceror gets all the spells at (ideally) level 17 and a Druid at 21, the quickest (level-wise) way to get all the spells would be to switch to a Druid as soon as possible, as long as you are able to do it before level 14. The disadvantage, of course, is that you have to go all that time without either class' high-level spells. Since experience is (in game terms) trivial to accumulate, it makes more sense to learn all the spells of one class before switching to either the other spellcaster or the Druid.
>
I disagree. I think Druids are most useful when they learn both sets of spells at the same time. If you're going to level a character up to level 17+ and retrain him anyway, you may as well only have to do it once. I wouldn't worry about not having the high-level spells for a little while. They're not all that useful until you're fighting seriously strong monsters, and by then the rest of your party should be able to handle them alone if need be.

> Disadvantages of the Druid are low HP and poor equipment => low AC (and WC but not worse than Sorcerors and not much worse than Wizards,) and low HP. (We'll discount the slightly slower learning as above.)

I don't think the WC, HP and AC handicaps make much of a difference. Druids should always be in the back row (HP and AC are no help there), and if they get breathed on often enough to kill them your party will be getting so severely beaten up that you'll have to cast whole-party healing spells anyway, so HP won't matter.

> Advantages are permanent innate AC increase and, it appears, fast SP gain.
>
And the Items of Druidity (predictably) enhance both Wisdom and IQ, so they're somewhat more versatile at spellcasting than Sorcerors or Wizards. But the big bonus is SP, enough of which can keep your party alive pretty well indefinitely.

> The question is which is a better progression:
>
> Knight -> [Wizard/Sorceror] -> [Sorceror/Wizard] -> Assassin
>
> or
>
> Knight -> [Wizard/Sorceror] -> Druid -> Assassin?
>
As explained above, I prefer non-caster->Druid->Assassin or, for those starting as caster, [Wizard/Sorceror] -> [Sorceror/Wizard] -> Assassin. But I don't think there's that big a difference, especially in long games.

> Specifically:
> Does the SP advantage create a worthwhile difference once you've gained at least 20 or so Assassin levels?
>
Your Assassin will only gain 1-2 SP per level, so unless you plan to get a few hundred SP before retraining it probably won't be of colossal use to your party. I've found the only real use for front-line fighters to have SP is for healing between battles, or resurrecting when they're the only ones left. But being in front, they're very rarely the only ones left after a fight.

> Is the HP disadvantage significant enough to cause hardship on the way to becoming an Assassin?
>
Not if you keep your Druid in the back row where he belongs. He won't get hit by anything HP-sensitive.

> Does the AC gain create a noticeable difference once you've gained at least 20 or so Assassin levels?
>
Your Assassin will gain AC faster than the Druid did. And there's an effective limit at 65 anyway, at which point you're invulnerable to physical attacks.

> Is there an advantage in accumulating SP and/or AC in earning more Druid levels after you know all the spells?
>
You can never have enough SP. Every time you gain enough SP to heal your whole party, you save yourself a trip back to town.

^v^:)^v^
FB